It is desirable to have an economical means of producing electroluminescent (EL) display modules that can be tiled together. A tiled display requires individual display tiles or modules that may be placed in close proximity to give the appearance of a continuous display. Of the four major flat panel technologies: plasma, liquid crystal, vacuum fluorescence and EL, the latter has the advantage that connections may, in principle, be placed anywhere on the module. This is possible because the device consists of one glass sheet with a series of solid state coatings. Liquid crystal, vacuum fluorescent and plasma modules require both front and rear sealing (usually glass) that restricts the connection paths to the module edges. In addition, a seal is required to prevent gas or liquid leakage at the edges of the module. The combination of seals and electrical connections makes it extremely difficult to tile modules especially at higher pixel resolutions.
Currently, X-Y matrix addressing is employed for EL displays and connections are made at the edges of the glass such as disclosed in U.S. Pat. No. 4,999,539 issued to Coovert et al. The surface area occupied by these connectors uses space near the peripheral edges of the module thereby producing a large gap between the display area and the edge of the substrate. Another drawback of conventional EL display modules is that many employ moisture barriers covering the entire back of the module for preventing moisture or other undesired ambient elements from entering the layered EL structure which are sealed around the perimeter of the glass substrate. These moisture barriers also result in a gap between the display area and the edge of the substrate. Thus, areas around the edge of the module used for connections and sealing do not allow for radiating display elements to exist in these areas. This causes the module dimensions to exceed the active area of the module which adds to module size, weight and cost.
U.S. Pat. No. 5,124,204 issued to Yamashita et al. briefly reviews a typical prior art EL panel comprising the display elements formed on a glass substrate. This prior art EL panel is sealed by a glass cover sheet bonded to the glass substrate with silicone oil, containing silica gel powder, being sealed between the display element and the cover sheet. This arrangement is both heavy and results in a significant gap between the edge of the glass substrate and the outer edge of the display element. Yamashita et al. reduces the weight problem by replacing the glass cover sheet with a polymer based moisture-proof sheet; however, the drawback of the edge gaps remains thereby precluding tiling together of multiple modules.
U.S. Pat. No. 4,266,223 issued to Frame discloses a display device comprising thin film transistor circuits for sequentially switching a brightness control signal to an array of individual display elements. The device comprises transversely-spaced or interdigitated metallic electrode finger arrays deposited onto a glass substrate and an electroluminescent layer deposited on top thereof which is then coated with an insulator and then contact electrodes. Transistor arrays are then deposited on top of these display elements. Transversely-spaced electrodes and thin film transistor circuits are difficult and expensive to manufacture.
It would therefore be advantageous to provide an electroluminescent display module comprising an EL layer structure which avoids the need for edge connections and edge sealing so that the active area of each module extends to the edge of that module such that no extra gap is visible between active areas of adjacent modules tiled together. It would also be advantageous in such a structure to use a transparent substrate with an EL structure that allows connections to the rear of the substrate whereby the light reaches the viewer through the transparent substrate from the front.